Direct-sequence CDMA (DS-CDMA) type radiotelephones require some form of power management scheme to control the output power level of the end user's radiotelephone's transmitter to control system interference at the input of the base site receiver. DS-CDMA is explained in detail in John G. Proakis, Digital Communications, 2nd Edition, pages 800-845. This power management, performed by the combination of the base station receiver, base station transmitter, and user radiotelephone (mobile), prevents a transmitting radiotelephone from using more of the system capacity than is needed for adequate communication from the mobile to the base station.
This power control is performed by two power control loops, an open loop for the initial estimate and a closed power control loop to correct this initial estimate. The radiotelephone, in performing the open loop control, uses the received signal strength indicator (RSSI) to estimate the signal path power loss from the base to the radiotelephone. The radiotelephone can then compensate the amount of power loss when transmitting back to the base station. Since the loss from the base station to the radiotelephone is not the same as the radiotelephone to the base station due to generally different and widely separated transmit and receive frequencies, this is just an initial estimate that must be corrected.
The correction, which modifies the original estimate, is accomplished by the base station transmitting power control bits to the mobile radiotelephone at typical rates of 800 to 1000 times per second. A power bit of 1 instructs the radiotelephone to increase power by one power step and a 0 instructs the radiotelephone to decrease power by one power step or vice versa depending on an agreed upon system protocol. One power step is a set value typically in the range of 0.5 to 1.5 dB. The closed loop control has a typical control range of .+-.24 dB. The total power control range is typically 80 dB for a 1 watt maximum power mobile.
A DS-CDMA radiotelephone's power amplifier must be at least class AB linear since the modulation for the system may be partly amplitude modulated for all modulation types and also to control transmitter splatter after the modulation bandwidth has been set by filters before the power amplifier. As is already known by one skilled in the art, the output of linear power amplifiers have to be controlled by some form of in-line attenuation and not by controlling the power supply voltage. This causes a problem in systems, such as the DS-CDMA radiotelephone system, where the power control required has a large range. This wide power control range can result in spurious radiation out of the circuits at a higher level than the desired conducted radiation, thus interfering with the desired level of the transmitted signals. This type of radiation is also difficult to shield adequately to keep it from interfering with other circuits in the radiotelephone. Additionally, there is a problem with conducted leakage around variable attenuator circuits that control the RF output level. There is a resulting need for a circuit that accomplishes the variable attenuator function without causing the spurious radiation or leakage problems.